The diiron-oxo proteins have active sites consisting of metal centers bridged by oxo or hydroxo groups usually supported by carboxylate bridges. This emerging class of metalloproteins includes proteins that perform a variety of functions in biology--dioxygen transport (hemerythrin), the conversion of ribonucleotides to deoxyribonucleotides (ribonucleotide reductase), phosphate ester hydrolysis (purple acid phosphatases), and oxygen activation (methane monooxygenase, stearoly ACP delta/9 desaturase). Building on our past record of modeling structural and spectroscopic properties of such sites, we propose to make functional models using tetradentate tripodal ligands or polydentate dinucleating ligands with phenoxo or alkoxo groups designed to bridge metal centers. Emphasis will be placed on designing complexes that would afford intermediates postulated in the activation of dioxygen by diiron centers and characterizing the spectroscopic and reactivity properties of such species. We propose to generate intermediates such as O2 adducts of diiron(II) complexes, alkylperoxo and hydroperoxo derivatives of iron(III), and species with Fe(III)FE(IV) and FE(IV)Fe(IV) formal oxidation states. These complexes will be characterized by X-ray crystallography whenever possible and by a variety of spectroscopic techniques such as NMR, EPR, UV-vis-NIR, Raman, Mossbauer, magnetism, and EXAFS. In some cases, rapid kinetic methods will be employed to trap short lived species. The reactivities of these transient complexes will be studied for their ability to oxidize a range of substrates to obtain data that can be compared with those obtained on iron porphyrin systems. Heterobimetallic complexes of symmetric and unsymmetric dinuclreating ligands will be synthesized and tested for their ability to perform hydrolysis reactions; the design of such catalysts will be guided by the different roles proposed for the individual metal centers in a bimetallic hydrolase.